1. Field of the Invention
This invention relates generally to orifice fitting mechanisms such as are placed in pipelines or other flow-way systems and which retain an orifice fitting in a flow passage to thus provide for controlled metering of the fluid flowing therethrough. More specifically, the present invention is directed to the provision of a cartridge type sealing unit for orifice fitting mechanisms which permits unitary installation and replacement of an orifice plate, together with an orifice plate sealing unit and the sealing elements establishing seals between the orifice plate and orifice plate sealing unit and between the orifice plate sealing unit and the body structure of the orifice fitting.
2. Description of the Prior Art
Orifice fittings have been manufactured and utilized for a number of years in connection with measurement of flowing fluid in pipelines and other fluid flow systems. As examples of conventional orifice fittings, reference may be had to U.S. Pat. Nos. 1,965,826; 1,996,192; and 2,688,987 of Daniel Industries, Inc., the present assignee hereof. In each of these patents, an orifice fitting is provided having a removable internal orifice plate sealing unit that supports an orifice plate in the flow passage of the orifice fitting. A mechanism is provided for transporting the orifice plate sealing unit from its operative position in the flow passage to a retracted position where the orifice plate sealing unit and orifice plate is removed from the flow passage of the valve and transported into an isolatable housing receptacle. The orifice fitting mechanism incorporates an intermediate valve mechanism having the capability of establishing an isolating seal between the body structure of the orifice fitting and the isolatable housing thereof. After the orifice plate carrier has been transported into the isolatable housing receptacle and the valve mechanism closed, the receptacle or chamber then may be vented without shutting down the fluid flow system. The orifice plate then may be removed from its sealing unit and replaced after which the sealing unit may again be transported into the flow passage for further measuring activity.
The orifice fitting mechanisms set forth in the above patents are of the more complex design which allow changing of the orifice plates without shutting down the fluid flow system. In other cases, orifice plate mechanisms are of more simple nature and simply incorporate an orifice plate body structure having a closure bonnet which is removable for replacement of an orifice plate sealing unit and the orifice plate in assembly therewith. In such simple orifice fitting mechanisms, it is necessary to depressurize the fluid flow system before the orifice plate bonnet is removed. Depending upon the circumstances involved, both types of orifice fitting mechanisms are in wide use at the present time.
Orifice fitting mechanisms are typically utilized to measure the flow of petroleum-based vapor. In many cases, the petroleum product involved is of severely corrosive and errosive nature, thereby requiring orifice plates to be replaced quite frequently in order to maintain optimum accuracy of flow measurement. In many cases, the orifice plate sealing unit can become severely corroded during use and must be replaced. Replacement of orifice plate sealing units can become a significantly time consuming and expensive operation when the flow system is subject to major down time thereby involving significant production losses. It is desirable therefore to provide an orifice plate mechanism that permits rapid changeout of the orifice plate sealing unit by means of simple and efficient, low cost servicing procedures.
In orifice fitting, as well as many other flow controlling products for petroleum-based fluids, elastomeric sealing elements are utilized to maintain effective seals between metal parts. Although elastomeric seals may take many other suitable configurations, it is typical to employ O-rings where desirable because of the low cost nature and effective sealing capability thereof. When orifice fittings are maintained under high pressure service, however, the elastomeric material from which O-rings and other such sealing elements are composed typically absorbs a significant quantity of highly compressed gas. When it is desired to remove the orifice plate from the orifice fitting mechanism, regardless of the type of orifice fitting, it is necessary to depressurize a fluid chamber in order to gain access to the orifice plate sealing unit for servicing operations. When the environment about the sealing elements is depressurized, the absorbed pressurized gas within the sealing members tends to cause the sealing member to swell. In many cases, depending upon the volume and nature of the elastomer, the sealing members can swell so rapidly that in a period of a very few minutes the orifice plate sealing unit can become tightly seized or locked in place by the expanded sealing members. After an orifice plate mechanism has become seized by gas expanded sealing elements, it becomes extremely difficult to accomplish removal of the orifice plate carrier mechanism. At times, the sealing elements themselves must be cut away in order to accomplish the repair operation. This is a time consuming and expensive procedure that frequently causes severe losses of production due to excessive down time for repair operations. It is therefore highly desirable that the orifice plate sealing unit contain a minimal amount of elastomer to minimize the degree of swelling and prevent such seizing of the orifice plate sealing unit, thus allowing for the complete removal of the orifice plate carrier mechanism as rapidly as possible.
It is desirable to provide an orifice fitting mechanism which has the capability of being simply and efficiently serviced by field technicians without requiring the provision of any special tools other than those necessary for gaining access to the orifice plate carrier assembly.